This invention relates to conductive materials and is particularly concerned with conductive materials to serve the purpose of a heating means.
It has long been known that materials that are electrically conductive can generate heat. This phenomenon has spawned a considerable number of heating means for an equally considerable number of different purposes.
There have been attempts hitherto to create electrical heaters in sheet or web form to serve a wide variety of purposes. However the most usable form of conductive material is in the form of carbon particles or carbon black embedded in or coated on a carrier or substrate. A known difficulty with such materials is a lack of consistency in the embedded or coated carbon particles or carbon black leading to a lack of uniformity in the heating effect across a sheet or web, and the presence of hot and cold spots. In an attempt to combat this, the temptation is to increase carbon density, but the greater the density or concentration of carbon particles or carbon black, the less pliable is the sheet or web until the point is reached where the material becomes brittle. This can be tolerated in a circumstance where the sheet or web can be held rigid on a support, but not in any circumstance where a greater or lesser degree of flexibility is required of the heating means.
One known material has solved this problem, a sheet of web with a surface layer of carbon particle or carbon black of such a density that uniform heating across the full width and length of the sheet with the complete absence of hot and cold spots is achieved but with retained substantial flexibility. That material is sold under the British, Community and US Registered Trade Mark INDITHERM.
With materials such as Inditherm®, it is convenient to provide a connection to a source of electrical power by providing conductive rails on the carbon layer along the length of a web or sheet at opposite edges, those rails being such as not to impair the flexibility of the sheet. Thus, it is known to provide rails such as by screen printing of an appropriate conductive ink, and suitably connecting the rail to lead extending to an appropriate power supply or mains electricity by way of and appropriate transformer.
In all of its forms of construction, the material known as Inditherm® provides exceptional heating means where the heat is distributed evenly across its full width and length with the complete absence of hot or cold spots.
In certain circumstances, sheets of Inditherm® need to be firmly attached to or around items that need to be heated, or to supports on which are placed items that are to be provided with heat. It might well be that to ensure the positive connection of a sheet of Inditherm® in place, it needs to be clamped, and the clamps may be needed at the edges of the sheet, with a risk, albeit remote, that the conductive rail at the edge of the sheet will be damaged to an extent to disrupt the supply of electrical power along the full length of the rail.
If a rail is damaged, and the connection to mains is lost, then so would be the heating effect over a part at least of the sheet. In some usages of sheets of Inditherm® that might not be critical, and a damaged sheet replaced at relative leisure. There are, however, other usages of sheets of Inditherm® where it is vital that the heating effect over part of or the whole of the sheet of Inditherm® is maintained.
The object of the present invention is to ensure the maintenance of a supply of electrical power over the full length of a heating means in flexible sheet form.